Motor control device



Oct. 9, 1945. s ELBERTY, JR 2,386,304

MOTOR CONTROL DEVI CE Original Filed Oct. 17, 1940 laoo k Moran sP FIELD AMPERES 0 I00 200 300 400 50a 60a Jemime I j E /4 a: ll ll l I14 I' I' III" I II &/a C F 4] la F Snventor EOBEBT \S-ELBEETY J8. y 68 CIttorneg Patented Oct. 9, 1945 MOTOR CONTROL DEVICE Robert S. liberty. in, Wamesboro. Pa.,

' to Lsndis Tool iginal application Octo 1942, Serial No. 445,018

4 Claim. (Cl. 172-239) Thi application is a application Serial No.

division of my copending 361,535, filed October 17, 1940, now Patent No. 2,302,304 granted November 17, 1942 and the invention disclosed herein relates to speed control mechanisms for use where the relation between angular speed and peripheral speed of a tool or work piece or other object is constantly changing.

An object of the invention is to provide a speed control mechanism for adjusting the angular speed of a rotating member so that the selected peripheral speed may be maintained substantially constant as the diameter of the rotating member changes.

A further object of the invention is to provide a speed control mechanism of the above type wherein the speed of a driving member is so controlled and modified as to maintain the peripheral speed of a driven member substantially constant as the diameter of the driven member changes or increases.

A further object is to provide an electrical circult for an electric motor whereby the speed of said motor may be adjusted by increments which are a fixed percentage of the preceding speed.

Figure 1 is a curve showing the relation between motor speed, field current and resistance in the field.

Figure 2 is a wiring diagram showing an improved type of circuit for use with two adjustable resistances.

Figure 3 is a diagram of a circuit having means for varying the range of peripheral speeds.

Figure 4 is an optional circuit for selecting and maintaining wheel speeds.

Conventional adjustable speed motors may be operated at a plurality of speeds diflering from one to another by a fixed number of revolutions. This type of speed control is satisfactory for the majority of applications. However, there are a great many mechanisms wherein it is desired to maintain a selected peripheral speed of a driven mechanism and wherein the length of the periphery is continually changing. It is also advantageous on some applications to change or adjust the peripheral speed. However, the change in R. P. M. per increment of regulating resistance must vary depending upon the diameter oi the driven member or else the increments of resistance must change for a given change in R. P. M. by an amount dependent upon the diameter of the driven member. The first method is preferred. Applicant has found that this result may be obtained-within close limits by using a circuit wherein two adjustable resistances, an-

Ilsignor Company, Waynesboro, Pa.

ber 17, 19, Serial No. Divided and "lb application June 3,

other resistance. and the motor field are connected in series multiple relation.

Series multiple circuits as such are not new, a few having been cited against the parent apperforming the function of the circuits described herein. The value of each resistance in these circuits had to be calculated with the object of combining in a given circuit to produce the desired effect in the operation of a motor. The conception of the result described and the working out of the means for obtaining that result represents a decided advance in the art of motor speed control.

The principal application of my invention has to do with maintaining a constant surface speed for a grinding wheel as the wheel becomes smaller due to wear and dressing. One application of my invention is the operation of facing oil a large disk on a lathe or similar machine where it is desired to maintain a constant work speed at the point of engagement between work and tool. There are numerous other uses for my invention; for example, in winding material on a reel or spool, it is often desirable to drive the apparatus with little or no dimculty. As applied to electrical apparatus, my invention need not.

be limited to speed control thru the motor field, but may be used with any form of adjustable Obviously the invention is a device for selecting and regulating surface speed of one member of a machine relative to the position of where such surface speeds vary depending upon the diameter of the member at the point where the speed is measured.

A typical motor curve having the points A, B, C, and D, indicated thereon is disclosed in Figure 1. This figure shows graphically the relation between motor speed, resistance and field current for a given motor. The portions A-B and 0-D show the speed range of the motor for extreme positions of adjustable resistance I 4. For a single rheostat control it will be noted that the variation in motor speed for a given change in resistance is proportionally greater at the lower speeds than at the higher speeds.

However, in the circuit shown in Figure 2, a resistance It is placed in the field circuit in series with the adjusting resistance H and in parallel with the regulating resistance 21. The purpose of the resistance is to act as a bleeder and to cause the regulating resistance to effect a difi'erence in speed which is a fixed percentage of the precedins speed rather than a fixed number of revolutions when the adjustable resistance II is cut into the speed. This circuit has been found' to give the best results.

1 The values of adjustable resistances II and 21 and resistance can be calculated to give a'very "close approximation of the when the motor is done by selecting the values of required speed range characteristics are known.

ll, 21, and 05 to meet conditions indicated on motor curve as points A, B, C, and D. For example, consider a motor having a field resistance R: of 180 ohms circuit or when it is set for any I and aspeed range of 600-1800 R. PUM. In the value of resistance I4 R27 the value of adjustable resistance 21, and R65 the value of resistance 05. wan adjustable resistance It cut out, resistance 85' has no effect since full voltage is then applied directly to adjustable resistance 21.

From Figure 1, the manufacturers curve for this particular motor, we find that for a speed 0&990 R. 1?. M., an external resistance value of 190 ohms isrequired. Therefore, the value of adjustable resistance 21 or R21 is 190 ohms. By cutting in all of Brent this time, the maximum speed of the motor, 1800 R. P. 101., may be obtained. With R14 all in the circuit and R21 all out, the motor speed will be 110.0 R. P. M., I: will be .636 amp., Ex 83 volts.- Under these conditions the voltage drop across resistance 50 will also be 83 volts. These figures may all be obtained from the motor characteristic asdisclosed by the manufacturers curve and the application of Ohm's law thereto. The voltage drop mm across the adjustable resistance it under the above conditions would be the difference between the line voltage, 230, and the voltage drop across the field, 83, or 147. From Ohms law:

for (A) 130 R14+R14Rs5-2'32 Rss=0 when both resistances are cut in, that is, with R21 equal to 190 ohms for 1800 R. P. M., the

manufacturers motor cates that the current law:

mime-13o or (B) 320 Ru-I-RuRss-33'l R5s=0 Equation A minus Equation B is 105 Rss=0 Therefore Rss=1.81 R14 Substituting and solving for R14 in either of the Equations A or B, gives a value of 160 ohms and therefore R55 is equal to 1.81x160 or 290 ohms. By following the above procedure the values of resistances I4, 21, and 55 may be determined for any motor. It has been determined that the w justable resistance i i,

4 is shown a circuit asaaeoa total resistance in the circuit of Figure 2 may he expressed as follows:

i= 1s+Ru( fif +RI The objects of this invention may be attained more or less satisfactorily by using any of a number of electrical circuits the resistances of. which would have to be calculated in a manner similar to that disclosed herein. For example, in Figure wherein the controlling rewith the field and in par- Each of the parallel cirsistances are in series allel with each other.

cuits consists of an adjustable resistance l0. and

8! respectively and another resistance 82 or 88 respectively inserted ahead of said adjustable resistances. Resistance 82 prevents resistance 8| from becoming inoperative when resistance is zero. Resistance 88 prevents resistance 80 from becoming inoperative when resistance 8! is zero. The circuits shown in Figures 2 and 4 have one thing in common, that is, they are series multiple circuits including the motor field, two adjustable resistances and additional resistance which may or may not be adjustable.

The circuit disclosed in with that shown in Figure 2, except that an ad.- justable resistance 60 is substituted for adjustableresistance 2?. Adjustable resistance 60 has two ranges ti and 02 and a switch 68 to shift from onerange to the other. The same result might be accomplished by using two independent resistances with a switch to change from one to the other. Figure 3 shows switch 03 in position to cut out portion 0! of adjustable resistance 60.

Operation In the operation of .my device a predetermined motor speed is selected by means of ad and in response to changes in the driven mechanism, adjustable resistance 2? will be shifted so as to change the motor speed by an amount which is a fixed percentage of the preceding speed. For example, at a speed of 1600 R. P. M. a given change in resistance 21 would change the speed by say 10%, or R. P. M., while at a speed of 800 R. P. M. the same change in resistance would change the motor speed only 80 R. P. M.

This result is obtained by the use of a fixed resistance 55 which has theeffect of modifying the values of the adjustablev resistances i4 and 21 depending on the respective adjustment of said resistances. Actually, resistance 55 may be adjustable but after its value has been deter mined there is no need for any change. For this reason it is more practical to use a hired resistance.

' Resistance 55 functions by dissipating a definite part of the electrical energy which would otherwise pass thru the field winding F of motor i8. The amount of energy so dissipated varies with the setting of the adjustable resistances. The result of using such a bleeder is that for a given rheostat or combination of rheostats a wider range of speeds may be obtained than with the rheostats by themselves.

I claim:

1. In a speed control mechanism, a shunt motor, a motor field winding, means for chang ing the speed of said motor by increments which are a fixed percentage of the preceding speed including an adjustable resistance for varying the strength of said motor .field, a bleeder re-' sistance in parallel with said adjustable re Figure 3 is identical sistance and means for selecting a series oi basic motor speeds to be varied by said adjustable resistance including a second adjustable resistance in series with said bleeder and said first adjustable resistance.

2.1:: a speed-control mechanism, a shunt motor, a motor field winding, a pair of adjustable resistances for controlling the speed of said motor, one of said resistances being used to select a predetermined motor speed, the other to vary the motor speed relative to said selected speed and means for causing said speed to vary by increments which are a fixed percentage of the preceding speed in response to equal increments of resistance regardless of said selected speed including a bleeder resistance in series with said selecting resistance and in parallel with said second resistance and said motor field.

3. In a speed control mechanism an electric circuit including a shunt motor, a motor field winding, a source of power for said motor, an adJustable resistance in circuit with said field winding for selecting any of a plurality of motor speeds, a second adjustable resistance for adlusting said motor speed relative to said selected speed and means to maintain a fixed percentage of speed change for a given chance of resistance by regulating the efiective rate of change of the total field resistance including a bleeder resistance in parallel with the motor field and said adjusting resistance and in series with said selectins resistance.

4. In a speed control mechanism an electric circuit including a shunt motor, a motor field winding, a source of power for said motor, an adjustable resistance in circuit with said field winding for selecting any of a plurality of motor speeds, a second adjustable resistance for adlusting said motor speed relative to said selected speed and means to maintain a fixed percentage of speed change for a given change of resistance by regulating the eifective rate of change of the total field resistance including a bleeder resistance in parallel with the motor field and one of said adjustable resistances and in series with the other of said adjustable resistances.

ROBERT 8. EIBERTY, Ja. 

